Pulp consistency, for instance, is measured in the processing industry by using a biaxial measuring device where the axes are within each other. The axes are pivoted elastically to each other, enabling a limited twisting between the axes. The twisting, which can also be expressed as a phase difference, can be few degrees at the maximum.
The axes affect each other by means of magnetic fields produced by electromagnets, and thus, when the first axis is rotated, for instance, by an electromotor in the pulp to be measured, the other axis rotates as well. At the ends of the axes there are different protrusions, the rotation of which the pulp to be measured tries to decelerate to the different extents by its consistency. This causes a torque between the axes, tending to increase the phase difference between the axes pivoted elastically. The aim is, however, to keep the phase difference constant during the measurement by controlling the amount of current to be supplied to the coils, whereupon alternating magnetic forces compensate for the torque between the axes accurately. Pulp consistency, which is proportional to the torque, can be determined by measuring the current supplied to the coils. Generally, shearing and friction forces, viscosity or consistency of fluids can be measured correspondingly.
In the prior art, the gap between the axes is sealed with an elastomer seal, such as an O-ring seal. The use of the O-ring is, however, related with problems. Especially when the phase difference of the axes increases and varies, the O-ring slides, which changes the friction between the axes in a non-predefined way. In addition, since the seal has to withstand temperature variations and possibly various chemicals in the process, the properties of the elastomer seal change in the course of time, which also changes the friction between the axes in an unknown manner. Since the friction between the axes, consisting of the friction between the seal and the axes and the friction inside the seal, affects the torque between the axes, occasional and/or non-predefined changes caused by the seal in the torque hinder the measurement of fluid properties considerably. If this affects the process control, the entire process to be controlled might be brought into a false state and the quality of the end product becomes poorer.